The number and variety of portable and mobile devices in use have exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries, and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
Most present day systems require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers, having a tight coupling between primary transmitter coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to or on top of the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
The Qi wireless power standard describes that a power transmitter must be able to provide a guaranteed power to the power receiver. The specific power level needed depends on the design of the power receiver. In order to specify the guaranteed power, a set of test power receivers and load conditions are defined which describe the guaranteed power level for each of the conditions.
Many wireless power transmission systems, such as e.g. Qi, supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver.
In many systems, such communication is by load modulation of the power transfer signal. Specifically, the communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which is detected by a change in the amplitude and/or phase of the transmitter coil current or voltage.
More information of the application of load modulation in Qi can e.g. be found in chapter 6 of part 1 of the Qi wireless power specification (version 1.0).
For load modulation, the power transfer signal generated from the transmitter inductor is accordingly used as a carrier signal for the load modulation introduced by the changes of the loading of the power transfer signal at the power receiver. In order to provide improved power transfer performance, it is of course necessary for the communication reliability to be as high as possible, and specifically for the bit or message error rate to be minimized. However, the load modulation performance depends on many different operating characteristics and parameters, including for example the frequency of the power transfer signal, the specific load values for different loads of the load modulation etc.
Accordingly, it can often be difficult to achieve optimal communication performance in a power transfer system using load modulation. This aspect is particularly critical as the performance is often a trade-off between communication performance and other operating characteristics and performance. For example, there is often a contradiction between the desires of optimal power transfer performance and of optimal communication performance. Such issues are often particularly critical for lower values of the coupling between the inductors of the power receiver and the power transmitter, and thus is particularly critical for applications wherein the distance between these may be increased.
An improved power transfer approach would accordingly be advantageous. In particular, an approach that allows improved operation, improved power transfer, increased flexibility, facilitated implementation, facilitated operation, improved communication, reduced communication errors, improved power control and/or improved performance would be advantageous. Especially, in many scenarios, it would be advantageous to improve communication at decreased coupling factors, such as e.g. occurs for increasing distances between the power receiver and power transmitter coils.